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Tomorrow I am supposed to start up my 2.5 RS, which has just undergone a USDM STi motor swap, along with a host of other power parts.

My car was previously tuned with RC 440cc injectors on a 10:1 compression ratio. I am curious to know just how far off, if at all, my A/F will be with the new compression ratio, which should be around 8.3:1. Obviously there is now more volume in the cylinder area, so maybe more fuel?

Will I have to undergo drastic changes in my A/F and timing maps, or the new CR not have that much of an effect off boost?

Thanks for any info. I'm just sort of curious where I should start with it.

THere will be no change off of boost with your fueling. Your pistons still sweep the same volume, and if you didn't change your heads at all, you will have the same amount of air entering as before, your compression is just lower. If the car is cold or hot, your startup might change a bit from before though.